One known method for current measurement involves measuring the voltage drop across a shunt resistor. Digital/analog converters (ADCs), which convert the analog measurement signal into a digital measured value, are frequently used in modern measuring and switching devices for the purpose of measurement. In this case, the following problems may arise:    a) The measurement signal has a high dynamic range, since small currents (<10% of the rated current) still have to be measured with an acceptable resolution and large currents (≧10 times the rated current) also still have to be detected.    b) Accuracy must not be inadmissibly impaired by the effect of ambient temperature.    c) The electronic components (ADC, processor etc.) need to be supplied with current. In this case, two (DC) decoupled power supplies are frequently required, since there are components both at a low potential (zero potential or artificial star point) and at a high outer conductor potential.    d) In addition to the outer conductor current, the outer conductor voltage must also frequently be measured against zero potential.    e) In the event of an overload or a short circuit, the main circuit must be disconnected.
The applicant's earlier specification DE 101 05 982 A1, which was unpublished at the priority date of the present application, discloses a method in which a measurement signal is at a higher potential than zero potential. Further, there is a measuring device that requires a supply current. The measurement signal occurrs in the form of an analog value and its measurement information is transmitted, after A/D conversion, in the form of a digital signal to an evaluation unit which is at ground potential. Further, after A/D conversion of the measurement signal, the digital signal produced providing the clock for modulating the supply current, which thus likewise performs the function of the carrier for the information content of the measurement signal.
In order to overcome the abovementioned problems, the following measures are taken in this case:    a) High dynamic response: analog/digital converters having a linear characteristic curve and a correspondingly high resolution (number of bits) are used.    b) Effect of ambient temperature on accuracy: (expensive) special alloys which have a low temperature coefficient are selected for the shunt.    c) The voltage is supplied directly to an ADC at a low potential using either voltage converters, i.e. transformers, or voltage dividers.